It is conventional to pass a gas into the working zone, through the laser working head, for the purpose, in particular, of protecting the oxidation fusion bath. To this end, it is possible to cause the gas to emerge from the working head through the exit window of the laser beam, proceeding either from the space adjacent to the mirror for deflecting the laser beam, as described, for example, applicants' Patent Application FR 89/08,634, in the name of the applicant, or from an axial conduit which opens in a downstream direction into this window (EP-A-300,458).